The present invention relates, in general, to pipe connectors, and more particularly to tool joints of a type having a pin element and a box element.
Offshore drilling for gas is carried out more and more into deeper water which have higher underground gas pressures. These drillings have to be serviced and repaired. However, no reliable tools, i.e. special drill pipes that can be screwed together to form a string of pipes, are commercially available. Changing drilling operations require a tool with a pipe connector to satisfy the following requirements:
high resistance against internal pressure and gasproof seal,
indifferent and robust configuration for rough handling and multiple repeated use of the same tool,
frequent torquing up, without loss of sealing function,
withstand high torque and thus have high fatigue strength under changing bending stress and high torque withstanding abilities for workover operations,
easy and quick assembly and disassembly,
easy and cost-efficient repair works.
Currently, offshore/workover operations are carried out with larger, gastight tubing-thread connections which, although suitable to withstand the high inner gas pressures, are highly susceptible to repair because they cannot be repeatedly connected together. Thus such connections can be used only a few times and incur high costs. Moreover, such connections are not constructed for changing bending stresses necessarily encountered during offshore workover operations as a result of wake, current or vibrations caused by turbulence, so that a re-use is greatly diminished in view of the slight strength against changing bending stresses.
Other proposals includes the application of flange joints. Installation and dismantling of flange joints is, however, time-consuming and costly.
Normally, tool joints are used to join pipe members of a drill pipe, in particular tool joints with an internal shoulder and an external shoulder, because the configuration with dual joint areas is able to withstand high torque loading during threaded engagement and thus high operating torque. The tool joint is, however, gas permeable when higher gas pressures are involved. Although, it is known to employ an additional sealing ring made, e.g., of TEFLON material, to realize a seal against inner gas pressure, such a sealing ring can be re-used only to a limited extent and is suitable only for lower pressures (company leafletxe2x80x94Mannesmann OCTGxe2x80x94xe2x80x9cPremium Test Drill Pipe for high pressure oil and gas wellsxe2x80x9d, no. 2, 1989).
International patent publication WO 96/03605 describes a pipe connection in the form of a tool joint. The drill pipe is made of a string of individual pipe members which are secured together by a threaded connection, called tool joint. Each individual drill pipe member includes a pin element at one end and a box element at the other end, so that successive individual members can be threaded to one another via the tool joint by the threaded engagement of confronting pin and box elements. The pin element has a tapered outer threaded section extending between an external shoulder and a pin end face, and a threadless section between the external shoulder and the outer threaded section as well as between the outer threaded section and the pin end face. Mating with the pin element of one pipe member is the box element of another pipe, whereby the box element has a tapered inner threaded section between an internal shoulder and a box end face and a threadless section between the internal shoulder and the inner threaded section as well as between the inner threaded section and the box end face. When screwed together, the confronting threadless sections of the pin and box elements in the area of the external shoulder are longer than the confronting threadless sections of the pin and box elements in the area of the internal shoulder. The external shoulder contacting the box end face during threaded engagement constitutes hereby the primary shoulder.
A comparable pipe connector is described in German Pat. No. DE 32 45 819 B1. The pipe connector is also configured as tool joint for drill pipes and has a similar configuration as the tool joint disclosed in international patent publication WO 96/03605, with the external shoulder representing the primary shoulder which is first activated during threaded connection. The length of the threadless sections in the area of the external shoulder is also greater than the threadless sections in the area of the internal shoulder. In addition, it is proposed to make the distance of the box end face from the internal shoulder greater than the distance of the pin end face to the external shoulder to thereby realize a gap, when the box end face bears against the external shoulder during threaded connection.
The pipe connectors as described in International patent publication WO 96/03605 and German Pat. No. DE 32 45 819 B1 have shortcomings because they are unable to meet the requirements that are demanded of pipe connectors for use in offshore/workover operations, and include high gasproof seals to withstand internal pressures of greater than 10,000 psi, high strength under changing bending loads, ruggedness in handling and frequent reusability, without adverse affect on the seal, as well as quick assembly and disassembly and easy and cost-efficient service work.
It would therefore be desirable and advantageous to provide an improved pipe connector which obviates prior art shortcomings and is also suitable for use in offshore/workover operations.
The present invention resolves prior art problems by providing a pipe connector, which includes a pin element having a tapered outer threaded section between an external shoulder and a pin end face, a first threadless pin section between the external shoulder and the outer threaded section, and a second threadless pin section between the outer threaded section and the pin end face; and a box element configured for threaded engagement with the pin element and having a tapered inner threaded section between an internal shoulder and a box end face, a first threadless box section between the internal shoulder and the inner threaded section, and a second threadless box section between the inner threaded section and the box end face, wherein, at threaded engagement of the box and pin elements, the first threadless pin section and the second threadless box section confront one another in the area of the internal shoulder and are defined by a first length, and the first threadless box section and the second threadless pin section confront one another in the area of the external shoulder and are defined by a second length which differs from the first length, wherein the external shoulder of the pin element forms a primary shoulder, when abutting the box end face during threaded engagement of the pin and box elements, wherein the pin end face is spaced from the external shoulder at a distance which is smaller than a distance between the box end face and the internal shoulder of the box element, wherein the first threadless box section and the second threadless pin section are at least twice as long as the second threadless box section and the first threadless pin section, wherein the second threadless pin section has a first region adjacent the pin end face, wherein the first threadless box section includes a groove and a sealing member, with the first region of the second threadless pin section abutting the sealing member, when the pin and box elements are threadably engaged, to thereby provide a metallic seal, wherein the sealing member includes first and second contact surfaces in axial spaced-apart relationship, wherein the first contact surface is disposed proximal to the internal shoulder and provides a seal seat, wherein the second contact surface is disposed distal to the internal shoulder and provides a support function, wherein the groove of the first threadless box section provides an increased elasticity and is arranged between the inner threaded section of the box element and the sealing member.
High torque loading is absorbed by also employing dual joint areas, i.e. external joint area (primary joint area) and internal joint area (secondary joint area); however, in order to additionally limit the load of the secondary effective internal joint area, the pin element has a long threadless section which carries a component of the metallic seal. In the following description, the xe2x80x9cthreadless nose section of the pin element at the internal joint areaxe2x80x9d will also be denoted as xe2x80x9clipxe2x80x9d or xe2x80x9csealing lipxe2x80x9d. The length of the lip is at least twice as long, preferably three times as long, as the threadless section adjacent the external shoulder. In this way, as the tool joint is increasingly torqued up, the external shoulder is proportionally under more stress whereas axial pressure loads in the lip, which reduce the strength against internal pressure, are kept low. The risk of flexure which may be accompanied by an adverse affect on the sealing function as a consequence of the extension in length of the lip can be counteracted by the provision of a support contact which reduces the flexure to an insignificant level.
For increasing the elasticity between the threaded section and the sealing zone, the box element has the groove which does not assume the function of a relief groove to decrease stress peaks, but primarily is provided to make this area more elastic to reduce the compression in the lip. In addition, the groove serves as reservoir for excess grease escaping from the threads, without adversely affecting the sealing system. The groove is so sized to allow use of chasers for implementing an effective process for fabricating the thread.
According to another feature of the present invention, the second threadless pin section has a slanted straight contact surface for interaction with the two contact surfaces of the sealing member, wherein the two contact surfaces of the box element have a convex, bulbed configuration. Through suitable selection of the radii of the bulbed contact surfaces of the box element as well as an optimization of the mutually influencing distances of the seal seat forming contact surface from the joint area and from the support-forming contact surface, it is ensured that in the seal seat a sufficient degree of constant surface pressure is effected which is substantially independent from fabrication tolerances of the overlap as well as from length differences between pin and box elements, and that fluctuations in the support-forming contact surface are absorbed. As an alternative, the bulbed contact surfaces may also be formed in the threadless section of the pin element, confronting the internal shoulder of the box element. In this case, the threadless section of the box element, cooperating with the bulbed contact surfaces, is configured with a straight, inclined surface.
Calculations and analyses have shown that the desired results have been attained, when selecting the parameters as follows.
a) The radius RD of the sealing contact surface is at least 1.25 times the mean radius RL of the sealing lip of the pin element.
b) The radius RS of the support-forming contact surface is about twice the radius RD.
c) The box and pin elements have a diametrical difference, wherein the contact surfaces of the box element define an overmeasure which is dependent on a material of the box and pin elements, wherein the overmeasure UD in the sealing contact surface is governed by the equation:
UD=z1xc3x97RLxc3x97"sgr"0.2/E,
wherein z1=1.6 . . . 2.4, "sgr"0.2 is the yield point, E is the modulus of elasticity.
d) The overmeasure US in the support-forming contact surface is governed by the equation:
US=z2xc3x97UD
wherein z2=0.5 . . . 0.7.
The sealing contact surface is spaced at a distance LID from the internal shoulder of the box and is governed by the equation:
LID=z3xc3x97UDxc3x97{square root over (RL/SL)}
wherein z3=1.2 . . . 1.5, SL is a mean thickness of the sealing lip.
f) The sealing contact surface is spaced at a distance LDS from the support-forming contact surface, and is governed by the equation:
LDS=z4xc3x97{square root over (RLxc3x97SL)}
wherein z3=0.5 . . . 0.8.
g) The support-forming contact surface is spaced at a distance LSG from an entry point of the outer threaded section, and is governed by the equation:
LSG=z5xc3x97LD
wherein z5=1.5 . . . 2.0.
f) The sealing contact surface is spaced at a distance LDS from the support-forming contact surface, and is governed by the equation:
LDS=z4xc3x97{square root over (RLxc3x97SL)}
wherein z4=0.5 . . . 0.8.
According to another feature of the present invention, the internal shoulder is configured as 90xc2x0 shoulder. This is advantageous because generation of radial forces is prevented during compression, which would have an adverse effect on the sealing system.